Reforming is a process generally known to the petroleum industry as a process for the treatment of naphtha fractions of petroleum distillates to improve their octane rating by producing aromatic components from components present in the naphtha feedstock. Reforming is a complex process and involves a number of competing processes or reaction sequences. These include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, dehydrocyclization of an acyclic hydrocarbon to aromatics, and hydrocracking of paraffins to light products boiling outside the gasoline range. In addition, the dealkylation of alkylbenzenes and the isomerization of paraffins occur in reforming processes. Some of the reactions occurring during reforming are not desirable owing to their deleterious effect on the yield of commercially valuable products or upon the octane of the products. For example, hydrocracking reactions produce light paraffin gases, e.g., C.sub.1 -C.sub.4, and reduce the yield of products boiling in the gasoline range.
The interest in catalytic reforming processes is fueled by a desire to improve the production (yield) of the gasoline fraction while concurrently increasing its octane, while also having sufficient catalytic activity to minimize the use of excessive temperature conditions for the dehydrocyclization process.
Several catalysts have been generally employed for catalytic reforming. Catalysts comprising platinum on chlorinated-alumina supports and Pt--X on alumina or chlorinated-alumina supports, where X is rhenium, iridium or tin, have been used for reforming naphthas. U.S. Pat. No. 4,370,224 discloses a multimetallic reforming catalyst comprised of platinum, iridium, copper, selenium and halogen, composited with an inorganic oxide support or carrier, preferably alumina. In addition, several patents have been issued for catalysts and/or processes employing zeolite-containing reforming catalysts. For example, several patents have disclosed the use of the zeolite mordenite in reforming catalysts, e.g., see U.S. Pat. Nos. 3,546,102; 3,679,575; 4,018,711 and 3,574,092. In addition, the use of ZSM-type zeolites in reforming catalysts and/or processes have been disclosed in U.S. Pat. Nos. 4,104,320; 4,417,083; 4,434,311 and 4,347,394. Further, the use of various forms of zeolite L is disclosed in U.S. Pat. Nos. 4,104,320, 4,447,316, 4,347,394 and 4,434,311. U.S. Pat. No. 4,417,083 discloses a process for the production of aromatic hydrocarbons in the presence of a two-bed process configuration employing a catalyst containing from 0.1 to 1.5% by weight of at least one metal selected from the group consisting of platinum, rhenium, iridium, tin and germanium, and containing sulfur in an atomic sulfur/metals ratio of from 0 to less than 1, supported on a crystalline, zeolitic aluminosilicate compensated by alkali metal cations, having a pore dimension larger than 6.5 Angstroms. The zeolite component is employed as a carrier. Among the zeolites that can be used are the Faujasites X and Y, the zeolite L and the zeolite omega.
Several chemical reactions occur during reforming. The most difficult of the desired reactions in reforming is the dehydrocyclization of paraffins and may be employed to evaluate a catalyst for its usefulness in reforming. The dehydrocyclization of paraffins containing six carbon atoms is one reaction carried out in reforming and is known to be relatively difficult. The ease of paraffin dehydrocyclization is known to generally increase with the number of carbon atoms present in the paraffin. Accordingly, an acidic reforming catalyst useful in forming aromatics from C.sub.6 paraffins would also be considered to be equal or more effective in the conversion of paraffins containing seven or more carbon atoms. This conversion of acyclic hydrocarbons to cyclized and dehydrogenated aromatic products produces valuable aromatic products having higher octane value than the paraffins from which they were formed. Thus, the octane of the gasoline fraction increases as a result of both the decrease in paraffins and as a result of the increase in the higher octane value aromatic products with minimum yield loss as compared with simple paraffin cracking.
Although the prior art catalysts for reforming and dehydrocyclization have included the use of Group VIII metals with chlorinated-alumina and, in some instances, selected zeolite materials, the prior art has generally not disclosed the use of molecular sieves as components with noble metal/chlorinated-alumina catalysts and has not disclosed the use of non-zeolitic molecular sieves as components in reforming/dehydrocyclization catalysts.
U.S. Pat. No. 4,440,871 discloses a class of crystalline silicoaluminophosphates denominated as "SAPOs". The SAPOs of U.S. Pat. No. 4,440,871 are disclosed to be useful for hydrocarbon conversion reactions, including reforming and dehydrocyclization. The discussion of the use of SAPOs in reforming is set forth at column 70, lines 39 to 46 and reads as follows:
"The SAPO catalyst compositions employed in hydrocracking are also suitable for use in reforming processes in which the hydrocarbon feedstocks contact the catalyst at temperatures of from about 700.degree. F. to 1,000.degree. F., hydrogen pressures of from 100 to 500 psig, LHSV values in the range of 0.1 to 10 and hydrogen to hydrocarbon molar ratios in the range of 1 to 20, preferably between 4 and 12."
The discussion of the use of SAPOs in dehydrocyclization is set forth at column 71, lines 25 to 32, and reads as follows:
"Dehydrocyclization reactions employing paraffinic hydrocarbon feedstocks, preferably normal paraffins having more than 6 carbon atoms, to form benzene, xylenes, toluene and the like are carried out using essentially the same reaction conditions as for catalyst cracking. For these reactions it is preferred to use the SAPO catalyst in conjunction with a Group VII non-noble metal cation such as cobalt and nickel."
The above disclosures generally refer to the use of the class of SAPOs of U.S. Pat. No. 4,440,871 as catalysts for reforming and dehydrocyclization. This general disclosure is supported by evaluation of representative SAPO samples for their first-order rate constant. The first-order rate constant (k.sub.A) is derived from a n-butane cracking experiment described at column 72, line 63 to column 73, line 30. The values for the first-order rate constants for SAPOs are set forth at column 73, lines 21 to 30. SAPO-5 is reported to have a k.sub.A of 1.4 and 7.4 for two preparative examples, SAPO-11 is reported to have a k.sub.A of 0.5 and SAPO-31 is reported to have a k.sub.A of 0.2. The meaning of the relative values of the first-order rate constants and their relationship to hydrocarbon conversion processes is not discussed. Further, U.S. Pat. No. 4,440,871 does not discuss the selection of SAPOs for the hydrocarbon conversion processes.
The instant invention relates to novel reforming and dehydrocyclization catalysts and processes wherein the catalyst is formulated using selected non-zeolitic molecular sieves, e.g., such as the silicoaluminophosphates disclosed in U.S. Pat. No. 4,440,871, as components in reforming catalysts.